1. Technical Field of the Invention
The invention relates to the field of integrated telecommunications systems comprising Global Positioning System (GPS) receivers within mobile terminals, and more specifically to a system and method for improving the cold start time of an integrated GPS receiver.
2. Description of Related Art
The cellular telecommunications industry has made phenomenal strides in commercial operations throughout the world. Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continually increasing percentage of new telephone subscriptions around the world. Growth in major metropolitan areas has so far exceeded initial expectations. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. This continual growth has revealed the existence of a considerable customer demand for value added services. Innovative solutions are required to meet these needs for product and service differentiation while maintaining high quality service without unduly raising prices. One such value added service is locating the geographic position of a mobile terminal.
Determining the geographical position of a mobile station within a cellular network or other Public Land Mobile Network (PLMN) has recently become important for a wide range of applications. For example, positioning services may be desired by transport and taxi companies to determine the location of their vehicles and to improve the efficiency of dispatch procedures. In addition, for emergency calls, e.g., 911 calls, knowing the exact location of a mobile terminal may be vital in ensuring a positive outcome in emergency situations.
Furthermore, positioning services can be used to determine the location of a stolen car, to identify home zone calls which may be charged at a lower rate, to detect hot spots in a micro cell, or to provide premium subscriber services, e.g., the Where Am I service. The Where Am I service facilitates the determination of, for example, the location of the nearest gas station, restaurant, or hospital to a mobile station.
One technique for determining the geographic position of a mobile station is to use the satellite-based Global Positioning System (GPS). GPS is a satellite navigation system that provides specially coded satellite signals that can be processed in a GPS receiver to yield the position, velocity and time of a receiving unit. Four or more GPS satellite signals are needed to compute the three-dimensional locational coordinates and the time offset of a receiver clock relative to a fixed coordinate system.
The GPS system comprises twenty-four satellites (not counting spares) that orbit the Earth in approximately twelve hours. The satellite orbits repeat virtually the same ground track once every day. The orbital altitude of the GPS satellites (20,200 Km) is such that the satellites repeat the same ground track and configuration over any point approximately once every twenty-four hours. There are six orbital planes each nominally with four satellites in each, that are equally spaced (i.e., 60° apart) and inclined at about 55° relative to the equatorial plane of the Earth. This constellation arrangement ensures that between five and eight satellites are visible to users from any point on Earth.
The satellites of the GPS system offer two levels of precision in determining the position, velocity and time coordinates at a GPS receiver. The bulk of the civilian users of the GPS system use the Standard Positioning Service (SPS) which has a predictable accuracy of ±100 meters horizontally, ±156 meters vertically and ±340 ns time accuracy. The Precise Positioning Service (PPS) is available to only to authorized users having cryptographic equipment and keys and specially equipped receivers.
Each of the GPS satellites transmit two microwave carrier signals. The L1 frequency (centered at 1575.42 MHZ) carries the navigation message as well as the SPS code signals. The L2 frequency (centered at 1227.60 MHZ) is used to measure the ionospheric delay by receivers compatible with the PPS system.
The L1 and L2 microwave carrier signals are modulated by three binary codes: a 1.023 MHZ Coarse Acquisition (C/A) code, a 10.23 MHZ Precise Code (P-Code) and a 50 Hz Navigational System Data Code (NAV Code). The C/A code is a pseudorandom number (PRN) code that uniquely characterizes a GPS satellite. All of the GPS satellites transmit their binary codes over the same L1 & L2 carriers. The multiple simultaneously-received signals are recovered by a Code Division Multiple Access (CDMA) correlator. The correlator in a civilian GPS receiver first recovers the C/A Code as modulated by the NAV Code. A Phase Locked Loop (PLL) circuit then separates out the C/A Code from the NAV Code. It should be emphasized that a GPS receiver first needs to determine its approximate location in order to determine which of the GPS satellites are within range. Conversely, a GPS receiver that knows its approximate position can tune faster into the signals transmitted by the appropriate GPS satellites.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. As explained elsewhere in this application, this process of initializing a GPS receiver may often take several minutes.
The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has. Most GPS receivers are programmed with almanac data, which coarsely describe the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver cannot correlate signals from the visible satellites fast enough, and therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is need for capturing the C/A Code and the NAV Code at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open becomes progressively harder to acquire when a receiver is under foliage, in a vehicle, or worst of all, in a building.
Recent governmental mandates, e.g., the response time requirements of the FCC Phase II E-911 service, make it imperative that the exact position of a mobile handset be determined in an expedited manner. Thus, in order to implement a GPS receiver effectively within a mobile terminal while also meeting the demands for expedited and accurate positioning, it has become necessary to be able to quickly provide mobile terminals with accurate assistance data, e.g., local time and position estimates, satellite ephemeris and clock information (which may vary with the location of the mobile station). The use of such assistance data can permit a GPS receiver that is integrated with or connected to a mobile station to expedite the completion of its start-up procedures. It is therefore desirable to be able to send the necessary assistance GPS information over an existing wireless network to a GPS receiver that is integrated with or connected to a mobile terminal.